Method of producing a forged article from prealloyed water-atomized ferrous alloy powder

ABSTRACT

Connecting rods for reciprocating internal combustion engines are forged from a mixture of graphite and a prealloyed iron metal powder comprising about 0.25 - 0.6 weight percent manganese, 0.2 - 1.0 weight percent nickel, and 0.2 - 0.8 weight percent molybdenum. Impact strength, fatigue properties and mechanical strength of the forgings are comparable to wrought alloy steels. Differential gears, driveshaft yokes, and other components requiring high tensile and impact strength and good fatigue properties also can be forged from the prealloyed powder.

0 United States Eatent 1191 1111 3,889,350

Mocarski June 17, 1975 [54] METHOD OF PRODUCING FORGED 3,605,245 9/1971 Zapf 29/420.5 ARTICLE FROM PREALLOYED 3,665,585 5/1972 Dunn et al. 29/420 3,720,5l2 3/1973 Yamaguchi et al... 75/226 X WATER'ATOMIZED FERROUS ALLOY 3,725,142 4/1973 Huseby 75/.5 BA x POWDER 3,795,129 3/1974 0010 29 420.5 x [75] Inventor: Stanislaw Moearski, Windsor, 3,834,004 9/l974 Ayers 29/420.5

Canada Primary ExaminerC. W. Lanham [73] Asslgnee. 11:40 :1 Motor Company, Dearborn, Assistant Examiner D' C Reiley, "I

Attorney, Agent, or FirmJoseph W. Malleck; Keith [22] Filed: Oct. 9, 1973 L. Zerschling [21] Appl. No.: 404,556

Related US. Application Data [57] ABSTRACT [62] Division of Sen 129 146 March 29 1971 Connecting rods for reciprocating internal combustion abandoned engines are forged from a mixture of graphite and a prealloyed iron metal powder comprising about 0.25 52 vs. C] 29/420.5; 29/DIG. 31; 75/5 AA; weight Percent manganese, weight P 75 5 17 22 cent nickel, and 0.2 0.8 weight percent molybde- 51 1m. (:1 B22f 3/24 Impact Strength, fatigue Properties and mechani- [58] Field of Search 29/420, 420.5, DIG. 18, Cal Strength Of the forgives are Comparable to wrought Z9/DIG 3 7 7 0; 75/5 AA, 5 BA 5 C, alloy steels. Differential gears, driveshaft yokes, and 5 123 J, 123 K 2 other components requiring high tensile and impact strength and good fatigue properties also can be [56] References Cited forged from the prealloyed powder.

UNITED STATES PATENTS 5 ChimsNo Drawings 3,528,081 9/1970 Huseby et al 75/.5 BA

METHOD OF PRODUCING A FORGED ARTICLE FROM PREALLOYED WATER-ATOMIZED FERROUS ALLOY POWDER This is a division of Application Ser. No. 129,146, filed Mar. 29, I971, and now abandoned.

SUMMARY OF THE INVENTION Prealloyed ferrous powders suitable for molding by conventional powder metallurgy techniques have been available for several years for use in forming a variety of articles and components. Prealloying elements are selected to provide sufficient hardening ability to permit raising the strength and hardness levels of the resulting components by heat treatment or other techniques. Relatively large amounts of nickel and molybdenum have been necessary to achieve satisfactory strength levels in the conventional prealloyed powders. Other prealloyed powders have included large amounts of manganese, although forgings from such powders have not achieved the strength and hardness levels expected from the known hardening properties of manganese because the manganese oxidizes readily and its oxides depreciate strength and hardness properties. Manganese thus has been avoided in recent investigations seeking prealloyed powders for high strength forgings. The high amounts of nickel and molybdenum required in such powders renders the use of such powders in forming relatively large components uneconomical. Primarily for this reason, the prealloyed powders traditionally have been used for relatively small components having complex but moldable shapes.

This invention is based on the discovery that forgings having high fatigue and strength properties can be produced from prealloyed ferrous powders-consisting essentially of small but balanced amounts of manganese, nickel and molybdenum. Low cost of the prealloyed powder renders it highly suitable for use in forging relatively large components such as connecting rods for reciprocating internal combustion engines, gears including the large automotive differential gear, and yokes for automotive driveshafts. The metal powder comprises a prealloy of about 0.25 0.6 weight percent manganese, about 0.2 1.0 weight percent nickel, and about 0.2 0.8 weight percent molybdenum. Oxygen content of the prealloyed powder is kept at a minimum, preferably less than 0.15 percent, and its carbon content also is maintained at low values, usually less than 0.01 weight percent. Desired strength and hardenability in the article forged from the powder can be obtained by adding effective amounts of graphite or other forms of carbon to the powder. Such graphite additions also improve the compaction of the powder and graphite mixture. In general, sufficient graphite is added to raise the carbon content of the gears forged therefrom to about 0.2 percent and of connecting rods forged therefrom to about 0.4 percent. Higher amounts of graphite are added for quenched and drawn high tensile articles, articles worked at high Hertz stresses and antifriction bearings. An amount of the added graphite that depends on the oxygen content of the prealloyed powder is lost during sintering so the actual amount of graphite added to the prealloyed powder usually is slightly higher than the amount desired in the final product. Natural graphite, for unknown reasons, produces the best results in this invention.

Suitable prealloyed powders can be made by water atomization of appropriate low carbon steel. The oxyv gen content of the steel is kept at low values by using a nozzle while pouring the molten steel from its ladle into a tundish and by maintaining the length of the poured stream at a minimum. If desired, the steel and its powders can be held in an inert atmosphere whenever elevated temperatures are used.

Prealloyed powders containing about 0.27 0.37 weight percent manganese produce the best fatigue strength and mechanical properties. Economics of strength and of fatigue are maximized in such powders with 0.40 0.48 weight percent nickel and 0.55 0.63 weight percent molybdenum.

Copper can be substituted for the nickel if desired. but hot shortness tends to result if the copper content exceeds the nickel content. Copper substitutions thus are limited to about 50 percent of the nickel. The best combination of economics, strength and fatigue is provided in prealloys containing about 0.20 0.35 weight percent copper, 0.20 0.35 weight percent nickel, 0.40 0.60 weight percent manganese and 0.20 0.30 weight percent molybdenum. Cobalt also can be added, although it has a negative hardenability factor. Other known alloying elements also can be added if desired.

Satisfactory connecting rods and differential gears have been produced with prealloyed powders having particle sizes ranging from 28 mesh to l00 mesh. Test data indicates that powders having particle sizes well beyond this range can be used for these and other articles. Connecting rods having equivalent strength levels but up to 12 percent lighter than conventional rods can be forged rapidly and efficiently from the powders. Weight reductions also can be realized by forging the powders into gears. Virtually all of the components forged from the powders of this invention require minimal machining.

Forgings are produced from the prealloyed metal powders by compacting the powder mixtures into approximately the desired shape, burning off any lubricants or binders, and sintering under a reducing atmosphere at about 2000F. Improved impact strengths can be achieved by sintering at higher temperatures such as 2200-2400F. Sintered compacts preferably are equalized at about 1600F under a reducing atmoshpere. The compacts at this stage typically have a porosity of about 10-25 percent. Forging is carried out on compacts initially at about l600F and at pressures of about 40-80 tons per square inch which reduces porosity to about one percent. In a mass production facility, forging can be carried out on compacts directly from and at the temperature of the equalizing oven. Forged articles are cooled under controlled conditions or quenched and drawn to provide the desired combination of mechanical properties. I-Ieat treated forgings produced from the powders have tension-compression endurance limits using the ASTM staircase method exceeding 50,000 psi unnotched and 35,000 psi notched with a k factor of 1.42, which shows that the materials are not notch sensitive.

The highly desirable properties of the prealloyed powders of this invention apparently result from the protection accorded to the interior of each powder particle by a surface layer rich in nickel and molybdenum. A thin scale of manganese oxide also forms on the particle surface and combines with the enriched nickelmolybdenum layer to protect the ingredients beneath the surface. Any iron oxides that might be present are reduced by hydrogen during the sintering operation;

manganese oxide cannot be reduced in this manner but the small quantities of manganese oxide formed on the surfaces of the particles coagulate during sintering into small globules that do not affect significantly the fatigue'and strength properties of resulting forgings.

DETAILED DESCRIPTION Example I particle size of about 28 mesh and the graphite is a very fine natural graphite. Ordinary trace amounts of phosphorus, sulfur, silicon, copper, and chromium are present'in the powder.

The resulting mixture is compacted under a pressure of about 30 tons per square inch and the compacts are passed through an oven operating at about 800-l 600F to burn off the lubricant. Under a hydrogen atmosphere, the compacts are sintered at a temperature of 2050F and cooled to room temperature. The resulting compacts have a porosity of about -20 percent.

Compacts are reheated under the hydrogen atmosphere to about 1700F, placed in a forging press and forged while approximately at that temperature at a pressure of about 40-80 tons per square inch. The forged articles are quenched and drawn to a hardness of about Rockwell C20. The resulting articles have a yield point of 89,000 pounds per square inch and a tensile strength of 108,000 pounds per square inch. Test specimens elongate 20 percent during tensile strength measurements and exhibit area reductions of about 50 percent. Charpy V-Notch impact tests (ASTM E- 23-66) produced impact strengths of 21 foot pounds at 212F and 10 foot pounds at 40F. Jominy hardenability tests (SAE 406a) produced a Rockwell C hardness of56 at Jl, 31 at J4, 28 at J8 and 22 atJl6.

Example 2 A prealloyed iron powder containing 0.47 weight percent manganese, 0.43 weight percent nickel and 0.55weight percent molybdenum with less than 0.15

' weight percent oxygen and about 0.01 weight percent carbon is mixed with sufficient graphite to produce a carbon content of 0.47 weight percent after sintering. The powder has a particle size of about 28 mesh and the graphite is very fine natural graphite. Test specimens producedby the procedure described in Example 1 exhibit a yield point of 88,000 per square inch and a tensile strength of 104,000 pounds per square inch. Test specimens elongate 22 percent during tensile measurements and exhibit area reductions of 40 percent. Charpy V-Notch impact strength at 32F is foot pounds. Jominy hardenability tests showed a Rockwell C of 57 at Jl, 46 at J4, 30 at J8 and 27 at J16.

Reducing the amount of manganese to about 0.30 weight percent and increasing the molybdenum to about 0.60 weight percent produces specimens with approximately the same tensile and hardness levels that have Charpy V-Notch impact strengths exceeding 40 foot pounds at 68F. Each of these materials is deemed to be suitable for forging connecting rods, gears and other automotive components.

Thus this invention provides a prealloyed iron powder that can be forged into articles having tensile and impact properties satisfactory for numerous automotive applications including relatively large gears, connecting rods, etc. Mixtures of the powder with appropriate amounts of graphite provide excellent levels of hardenability.

I claim:

1. A process for forging an article having high impact strength from a metal powder comprising water atomizing a charge of molten ferrous alloy metal to a prealloyed iron metal powder consisting essentially of about 0.25 0.6 weight percent manganese, 0.2 1.0 weight percent nickel, and 0.2 0.8 weight percent molybdenum, said prealloyed powder containing minimal amounts of carbon and oxygen the balance being substantially iron,

mixing said powder with an effective amount of graphite to provide for desired hardenability, and

forging said powder and graphite mixture into the shape of the article.

2. The process of claim 1 comprising compacting the powder and graphite mixture into approximately the desired shape of the article,

sintering the compacted mixture in a reducing atmosphere, and

hot forging the compacted mixture into the shape of the article.

3. The process of claim 2 in which the sintering step is carried out at least at about 2000F and the forging step is carried out at an initial temperature of about 1600F.

4. The process of claim 3 comprising equalizing the compacted mixture leaving the sintering step at about l600F in a reducing atmosphere, and forging the compacted mixture directly after equalizing.

5. A process for forging an article from a ferrous based metal powder, the article having an impact strength of at least 10 foot-lbs. at 40F. and a jominy hardenability of at least 56 at J1, 31 at J4, 28 at J8, and 22 at J16, the method comprising:

a. water atomizing a charge of molten ferrous alloy metal to form a prealloyed metal powder consisting essentially of 0.25-0.6 weight percent manganese, 0.2-1.0 weight percent nickel, 0.2-.8 weight percent molybdenum, less than 0.1 weightpercent carbon, a minimum amount of oxygen, the remainder being substantially iron,

b. mixing said water atomized powder with an amount of graphite effective to provide a carbon content in said article of at least 0.4 weight percent,

c. forging said powder and graphite mixture at an elevated temperature of about 1600F. to form the shape of said article, and

d. quenching and drawing said article to a hardness of about R 20. 

1. A PROCESS FOR FORGING AN ARTICLE HAVING HIGH IMPACT STRENGTH FROM A METAL POWDER COMPRISING WATER ATOMIZING A CHARGE OF MOLTEN FERROUS ALLOY METAL TO A PREALLOYED IRON METAL POWDER CONSISTING ESSENTIALLY OF ABOUT 0.25 - 0.6 WEIGHT PERCENT MANGANESE, 0.2 - 1.0 WEIGHT PERCENT NICKEL, AND 0.2 - 0.8 WEIGHT PERCENT MOLYBDENUM, SAID PREALLOYED POWDER CONTAINING MINIMAL AMOUNTS OF CARBON AND OXYGEN THE BALANCE BEING SUBSTANTIALLY IRON, MIXING SAID POWDER WITH AN EFFECTIVE AMOUNT OF GRAPHITE TO PROVIDE FOR DESIRED HARDENABILITY, AND FORGING SAID POWDER AND GRAPHITE MIXTURE INTO THE SHAPE OF THE ARTICLE.
 2. The process of claim 1 comprising compacting the powder and graphite mixture into approximately the desired shape of the article, sintering the compacted mixture in a reducing atmosphere, and hot forging the compacted mixture into the shape of the article.
 3. The process of claim 2 in which the sintering step is carried out at least at about 2000*F and the forging step is carried out at an initial temperature of about 1600*F.
 4. The process of claim 3 comprising equalizing the compacted mixture leaving the sintering step at about 1600*F in a reducing atmosphere, and forging the compacted mixture directly after equalizing.
 5. A process for forging an article from a ferrous based metal powder, the article having an impact strength of at least 10 foot-lbs. at -40*F. and a jominy hardenability of at least 56 at J1, 31 at J4, 28 at J8, and 22 at J16, the method comprising: a. water atomizing a charge of molten ferrous alloy metal to form a prealloyed metal powder consisting essentially of 0.25-0.6 weight percent manganese, 0.2-1.0 weight percent nickel, 0.2-.8 weight percent molybdenum, less than 0.1 weight percent carbon, a minimum amount of oxygen, the remainder being substantially iron, b. mixing said water atomized powder with an amount of graphite effective to provide a carbon content in said article of at least 0.4 weight percent, c. forging said powder and graphite mixture at an elevated temperature of about 1600*F. to form the shape of said article, and d. quenching and drawing said article to a hardness of about Rc20. 